Unregistered role is good to prevent unauthorized access and put clients to the different VLANs.
But in the real world we faced that most of Android phones and some other devices cant properly change IP address, and users has bad experience.
So Adtran should add ability to put clients directly to desired VLAN mapped to SSID with restricted access and redirect to the Captive portal, and after successful authorization open access. As many other vendors do.
I believe Adtran would do that and some other features, because now it is good solution for ISP and after some improvements it will be great